(1) Field of the Invention
The present invention relates to a rubber composition suitable for pneumatic tires which concurrently achieves satisfactory rolling resistance, wet skid resistance, breakage resistance and wear resistance.
(2) Description of the Prior Art
Recently, resource saving and energy saving have been socially demanded and investigations for developing so-called low fuel consumption tires have been eagerly carried out in order to reduce power loss is reduced.
It has been generally known that an automobile provided with a tire having a low rolling resistance is low in gasoline consumption. This tire is a so-called low fuel-consumption tire. In order to lower the rolling resistance, materials generally used have a low glass transition temperature, such as cis-polybutadiene rubber and the like, have a low hysteresis loss, such as natural rubber and the like, as a tread rubber.
However, these rubbers are extremely low in the running stability, such as braking performance on a wet road and wet driving performance. It has been very difficult to concurrently achieve a satisfactory running stability on a wet road along with a satisfactory rolling resistance.
Recently, it has been known, as described in Japanese Patent Laid-open Application No. 62,248/79 (corresponding to U.S. Pat. No. 4,334,567), to improve rolling resistance and running stability by using only styrene-butadiene copolymer (SBR) having a styrene content of 20-40% by weight and a 1,2-bond content in the bonded butadiene of 50-80% by weight, but all of the polymer consists of said SBR, so that satisfactory results have not always been obtained. Thus, since all of the polymer consists of said SBR, running stability is improved over prior SBR but the rolling resistance is not yet satisfactory. Moreover, in this prior art, conventional solution polymerization-type SBR obtained by using organolithium compound is used. Therefore, a rubber composition containing the SBR is poor in the breakage strength and in the elongation at break. Hence various problems arise in the running of the tire or in the production of the tire. The former problem is the formation of cuts and cracks on the tread surface during the running of the tire on nonpaved road. The latter problem is the breakage of the tread caused by the mold when a vulcanized tire is taken out from the mold, that is, so-called tread scratch formed by the mold. Furthermore, the above described SBR has a glass transition temperature higher than that of conventional SBR, and therefore the rubber composition containing the above described SBR is apt to harden during running in a cold region or in winter. The tire tread often fails due to the low-temperature brittleness.
Accordingly, a rubber composition concurrently achieves satisfactory rolling resistance and running stability while maintaining excellent breakage and wear resistances has not been developed.
The inventors have diligently tried to solve the above described drawbacks and noticed, as described in U.S. patent application Ser. No. 231,529, now abandoned, that the deformation at the friction surface of the tread occurs at high speed, and the hysteresis loss in the deformation at high speed has a great influence on the friction force, that is, the running stability of the tire, while the deformation of the tread owing to ground contact, which has a great influence on the rolling resistance of the tire, corresponds to the rotation speed of the tire, so that the hysteresis loss at the deformation of less than 100 Hz has a great influence on the rolling resistance. According to the temperature-time deducibility by Williams, Landel and Ferry, it is supposed that the hysteresis loss at high speed controlling the running stability conforms to the hysteresis loss measured at a lower temperature than the temperature at which the tire is used, and hence the inventors have made study concerning the condition under which the hysteresis loss is made to be larger in order to improve the running stability and found that when the hysteresis loss measured at about 0.degree. C. in the dynamic measurement of 1.5 Hz is larger and the hysteresis loss at 50.degree.-70.degree. C. is smaller, rubber compositions wherein both the running stability and low rolling resistance are concurrently improved, can be obtained.
Based on the above described discovery, the inventors have disclosed in U.S. patent application Ser. No. 231,529 that high vinyl SBR having a 1,2-bond content in butadiene unit of not less than 60% by weight can somewhat improve concurrently the low rolling resistance and running stability. However, since such high vinyl SBR is insufficient in the breakage resistance and wear resistance, the high vinyl SBR must be blended with other diene rubber. Moreover, unless a large amount of other diene rubber is blended, the resulting rubber cannot be practically used. As the results, when high vinyl SBR is used, the resulting rubber composition is satisfactory in the rolling resistance, but is poor in the running stability.
The inventors have further disclosed in U.S. patent application Ser. No. 251,843, now abandoned, that a rubber composition consisting mainly of an amorphous gradient high vinyl butadiene-styrene copolymer rubber having an average content of 1,2-bond in butadiene unit being not less than 60% by weight, in which copolymer the 1,2-bond content uniformly varies along the molecular chain of the copolymer, can improve the running stability and low rolling resistance of a tire while maintaining the breakage strength of the tire.
However, the high vinyl SBR disclosed in U.S. patent application Ser. Nos. 231,529 and 251,843 is compatible with natural rubber and synthetic isoprene rubber, but is not compatible with polybutadiene rubber and ordinary styrene-butadiene copolymer rubber. Therefore, the high vinyl SBR cannot be mixed with all of the general purpose rubbers, which are widely used in the tire industry, in the same mixing ratio. When polymers constituting a rubber composition are not compatible with each other, the shape of the tan .delta.-temperature curve of the rubber composition is varied due to the variation in the kneading operation during the production of tire, to cause variation of tan .delta. at 0.degree. C. As a result, running stabilities, such as wet skid resistance and the like, are varied in every tire due to the difference in the production step.
While, an A-B type block BR consisting of blocks having different vinyl bond contents, and an A-B type block SBR consisting of blocks having different bonded styrene contents and having different vinyl bond contents in the butadiene unit, which are disclosed in Japanese Patent Application Publication No. 37,415/74, have improved wet skid resistance and low rolling resistance.
However, although such block BR has improved wet skid resistance and low rolling resistance, the block BR is not compatible with all of the general purpose rubbers. Therefore, the use of the block BR in a rubber composition for tire tread is not satisfactory in view of the unstableness of tire performances, such as wet skid resistance and the like, and the poor breakage strength, low-temperature brittleness and the like in the rubber composition.
However, the running stability of a tire on a wet highway road surface becomes a more and more important property of the tire in view of safety, and the development of low fuel consumption tires having high performance has been basically for resource saving. Bearing these problems in mind, the inventors have made various investigations with respect to styrene-butadiene copolymer rubber in order to further improve the breakage strength, low rolling resistance and running stability of tire, and ascertained that the above described various properties can be improved by the use of a specifically limited rubber composition consisting mainly of a rubber containing at least 20 parts by weight, based on 100 parts by weight of the rubber, of a block copolymer having a specifically limited microstructure, and accomplished the present invention.